1. Field of the Invention
The present invention relates to light source units and light irradiation units, and device manufacturing methods, and more particularly to a light source unit that generates light of a desired wavelength by converting the wavelength of light amplified by an optical amplifier, a light irradiation unit that comprises the light source unit, and a device manufacturing method that uses the light irradiation unit in a lithographic process.
2. Description of the Related Art
Light irradiation units are conventionally used for fine structure inspection of objects, fine processing of objects, and for vision correction treatment. For example, in a lithographic process for producing semiconductor devices or the like, in order to transfer a pattern formed on a mask or a reticle (hereinafter generally referred to as a ‘reticle’) onto a substrate such as a wafer on which a resist or the like is coated or a glass plate (hereinafter appropriately referred to as a ‘substrate’ or a ‘wafer’) via a projection optical system, exposure apparatus are used, which is a type of a light irradiation unit. As such an exposure apparatus, a static exposure type projection exposure apparatus that employs a step-and-repeat method, or a scanning exposure type projection exposure apparatus that employs a step-and-scan method is mainly used. In addition, for vision correction, a laser vision correction system, which is also a type of light irradiation unit, is used to perform ablation of the corneal layer (PRK: Photorefractive Keratectomy) or ablation of the inner cornea (LASIK: Laser Intrastromal Keratomileusis) for treatment of nearsightedness, astigmatism, or the like.
Many light sources that generate light having a short wavelength have been developed for such light irradiation units. The direction of development of such light sources with short wavelengths can be mainly divided into the following two groups: development of an excimer laser light source whose laser oscillation wavelength itself is short, and development of a short wavelength light source that makes use of harmonic generation of infrared or visible light laser.
Of such development, along the direction of the former group, a light source unit that uses a KrF excimer laser (wavelength: 248 nm) has been developed, and at present, a light source unit that uses an ArF excimer laser (wavelength: 193 nm) is being developed as a light source having shorter wavelength. However, such excimer lasers have their disadvantages as light source units, such as their large size, and their complicated maintenance operation and high running cost due to the hazardous fluorine gas used.
Therefore, a method of shortening the wavelength along the direction of the latter group is gathering attention, which is a method of converting long wavelength light (such as infrared light or visible light) into ultraviolet light with a shorter wavelength by using a nonlinear optical effect of a nonlinear optical crystal. As a light source that uses such a method, details are available in, for example, International Publication WO99/46835 (hereinafter simply referred to as a ‘conventional example’).
In the wavelength shortening method using nonlinear optical crystals as in the method described above, the generating efficiency of short wavelength light depends on the generation efficiency of the nonlinear optical effect of the nonlinear optical crystal, however, in the current state, the usable generation efficiency of the nonlinear optical effect of the nonlinear optical crystal is far from high. Therefore, in order to obtain high luminance ultraviolet light, a highly intense infrared light or visible light has to be incident on the nonlinear optical crystal. So, in the above conventional example, a structure is employed to amplify an infrared light or a visible light of a single wavelength generated by a semiconductor laser or the like, with an optical fiber amplifier that has an amplifying optical fiber in which a rare earth element is added, and to make it enter the nonlinear optical crystal.
In such an arrangement, for example, in order to continuously (including an optical pulse train whose pulse interval is sufficiently short compared with an pumping saturation time of an amplifying optical fiber in an optical fiber amplifier) emit ultraviolet light of high intensity during the exposure period of shot areas in the exposure apparatus, exciting light with high intensity needs to be continuously provided.
When light subject to amplifying does not enter the amplifying optical fiber over a period of time exceeding the pumping saturation time of the amplifying optical fiber, such as during a stepping period in the exposure apparatus, while the exciting light with high intensity is being continuously provided, and then the light subject to amplifying enters the amplifying optical fiber for exposure of the next shot area, light of an extremely high intensity (a so-called ‘giant pulse’) is generated instantly in the amplifying optical fiber immediately after its entrance. And, when such a giant pulse is generated, it had the potential to affect the amplifying optical fiber, as in damaging its fiber arrangement (a predetermined core/cladding structure and a distribution of additive elements) or the like.
In order to prevent the giant pulse from being generated, controlling the exciting light intensity can be considered, however, the responsiveness of optical amplifying ratio control by controlling the exciting light intensity is not necessarily high. Therefore, in order to stabilize the luminance of irradiation light required in an exposure apparatus or the like while preventing the structure of the amplifying optical fiber from being damaged, a complicated and sensitive control of exciting light intensity had to be performed.